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Community Member

Control word to transport layer2 encapsulations

Hi,

what is the use of the control word when transporting layer2 enacpsulations across the network say MPLS,when it would be desirable ??

Thanks

Anand

6 REPLIES
Purple

Re: Control word to transport layer2 encapsulations

One of the reasons that you would use the control word is when you wanted your packets to be sequenced. In such a case, the control word would include a sequence number. For example, Structure-Agnostic TDM over Packet (SAToP) uses the Control word for that purpose.

In addition, you would use the control word when the PSN is an MPLS network, which inspects the MPLS payload in order to perform equal-cost load-sharing. The intent here is to prevent the payload appearing to be an IP packet. If it appeared as an IP packet, the router could decide to load-share the traffic, and that would be bad for something like circuit-emulated traffic. RFC4385 describes that scenario.

Hope that helps - pls do rate the post if it does.

Paresh

Community Member

Re: Control word to transport layer2 encapsulations

Hi Paresh,

As per "draft-martini-l2circuit-encap-mpls-07.txt"

In most cases, it is not necessary to transport the layer 2 encapsulation across the network; rather, the layer 2 header can be stripped at R1(Ingress), and reproduced at R2(Egress).See stripping layer2 header at R1 is fine,but at the same time we are adding control word so as to reproduce the same(original header) at R2,so my point is what kind of differnce does this make ??

Thanks

Anand

Bronze

Re: Control word to transport layer2 encapsulations

The control word is used to carry "stuff" like for instance in frame-relay the control word carries the FECN/BECN details. Within ATM it carrys ECGN etc. In ethernet at present it is not used and remains blank but there are movements in the MEF etc to have this used to carry Ethernet OAM details.

HTH

Community Member

Re: Control word to transport layer2 encapsulations

Robert,

whats the point in stripping layer2 header from the frame at ingress router and adding control word to the same to reproduce it at Egress router??

Juniper does it by default(sending control word),while Cisco does not.

Regards

Anand

Bronze

Re: Control word to transport layer2 encapsulations

ok two things from your statement there. Cisco do indeed send the control word by default. Its not a knob that can be turned off or on im afraid. What is the reason for stripping the layer 2 header? Well lets reverse that question into what is the point in not stripping the layer 2 header - in a point to point world who cares what the layer 2 header is it coming in one port and defo going out the other port. The control word as already stated keeps things like atm and frame paramaters intact only just in case they need to be used deeper into the customers network.

HTH

Community Member

Re: Control word to transport layer2 encapsulations

Robert,

We have Cisco PEs of 7613 series and Juniper PEs of M40e,the VCs through these PEs will come up only when we turn off the control word on juniper.I m afraid when you say Cisco do indeed send the control word by default.As far as layer2 header is concerned ,whats harm in keeping frame parameters intact by sending layer2 header itself without controlword .

regards

Anand

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